In IMS (Internet Protocol Multimedia Subsystems) based communications networks, communication sessions are established for transmitting information between different communicating parties using communication terminals. In this description communication terminals, such as e.g. mobile phones, are associated to IMS based communications networks, and fixed telephones are associated to the IMS based communications networks via one or more access nodes. However, throughout this document, the term “communication terminal” will be used for communication terminals of any type suitable for being associated directly to IMS based communications networks, such as e.g. computers, mobile terminals, as well as for communication terminals which are suitable for being associated to IMS based communications networks indirectly via any suitable access node such as e.g. fixed telephones. Such access nodes may typically be realised as e.g. gateways.
Typically, in IMS based networks information is transmitted according to the Session Initiation Protocol (SIP), which is a well known standard. SIP is therefore not necessary to describe in more detail here.
With reference to FIG. 1a, which illustrates a scenario according to prior art, a calling party (not shown) uses a communication terminal 100 for establishing a communication session via an IMS based communications network 102 with a called party (not shown) using a fixed telephone 104. The calling communication terminal 100 associated to the IMS based communications network 102 requests for a service from an application server 106, e.g. to establish a communication session with another communication terminal 104. The called communication terminal 104, being associated to a non-IMS based communications network 108, will not able to request for a desired service directly from another application server 110 in the IMS based network 102, but will be associated to the IMS based network 102 through an access node 112, e.g. a gateway.
In general, even if a communication terminal 100 is associated directly to an IMS based communications network 102, any suitable conventional access node 114, such as e.g. a P-CSCF (Proxy Call Session Control Function) will be applied to associate the communication terminal 100 to the IMS based communications network 102. Typically, the described IMS based communications network 102 may also comprise additional conventional means to be able to operate properly, e.g. for associating the communication terminals and/or access nodes to the IMS based network 102 and for establishing the connection between the application servers 106, 110. However, for simplicity reasons, any means or functionality which is not necessary for the understanding of the described scenario has been omitted in the figure.
When establishing a communication session one of the communicating parties may be busy, or limited resources may be available for incoming communication to the party. “Call Waiting” is a service that exists today in communications networks having knowledge whether the communication parties are available for incoming calls or busy in an on-going call, for instance in ISDN/PSTN (Integrated Services Digital Network/Public Switched Telephony Network). Typically, a called party busy in another communication session or a call receives a “Call Waiting” tone when a calling party wants to initiate a call. The called party can then either toggle to the calling party, end the on-going session or call and pick up the incoming call, or notify the calling party that he is already busy in another call.
“Call Waiting” is a desired service also in IMS based networks, and for a communication party establishing a communication session with another communication party it is desired to be able to use the same services that he/she is accustomed to, irrespective of which type of communications network applied. Generally, the communication parties do not always even have knowledge regarding which type of communications networks being applied for the communication session.
However, there are some problems associated with achieving Call Waiting functionality in IMS based communications networks. IMS based communications networks do not have knowledge of whether the called party is busy in another communication session and/or have limited resources for incoming communication. Moreover, an application server in an IMS based communications network does not have knowledge of how many terminals a called party has and/or can handle. In addition, it does not even know which type of terminal(s) the called party uses. Today, it is also not even possible for the IMS based network to know if “Call Waiting” is a desired service to the called communicating party, i.e. if the called party wants to be notified regarding an incoming call during an on-going call.